I have tried both a Jacobs and an MSD on my M3. Both worked well and likely so will some other spark boxes too. I couldn't find extra high RPM power (at least not yet) but so far have found improved output below about 4000-4500 RPM. Mostly things like driveability, flat spots etc were improved. Part throttle, cruise, throttle blip benefits. Also I can run an 0.045 gap on the cold Bosch XR4cs race plugs on the street without trouble. Tech note...rich mixtures are fairly easy to light off as there wil be many mixture molecules between / near the plug ends. Lean mixtures tend to prefer wider gaps cuz there are more chances to ignite the mixture since the wider gap requires more voltage and can "find" some mixture better than narrow gaps. Most modern high output motors use much wider than S14 stock gaps these days. I also ran better than stock plug wires and vented the dist cap (cuts ozone and therefore errant sparks in the cap) to improve the capacity to handle high voltage. Ignitions can be one of those kind of issues..like motor oil. Some feel strongly about one POV or another and both sides can be very persuasive.
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